The underlying hypothesis of this project is that the circadian clock allows cells to temporally separate metabolic pathways for optimal use of resources-in the case of the cyanobacteria, this means both internal metabolites and sunlight for photosynthesis. The project will exploit a well-studied cyanobacterium, Synechococcus sp. strain PCC 7942, which is the simplest organism known to have a bonafide circadian clock. Experiments are directed both at understanding how circadian gene regulation is accomplished and at determining the physiological consequences of circadian gene regulation is accomplished. Mutants that are defective for a specific sigma factor gene (sigC) show different periods when different when different reporter genes are monitored. This result challenges current clock models for control of downstream functions by a single oscillator. Experiments will test models for the following hypotheses: (1) the abundance and/or the activities of the cyanobacterial genome, and (2) there is more than one oscillator in the cyanobacterium; these may be overlapping oscillator and one or more non-Kai slave oscillators. Another goal is to determine whether carbon assimilation (photosynthesis) and carbon utilization pathways are temporally separated and, if so, whether this is important for the fitness of the cell. Transcript profiling will elucidate the temporal program of the both a daily light-cark cycle (to mimic the natural environment) and continuous light (to measure circadian control), will be used to create probes for DNA arrays that represent the genome. These experiments will address the fitness advantage of strains whose endogenous period matches the environmental cycle can be explained by appropriate phasing of critical cellular functions.